1. Field of the Invention
This invention relates generally to instrumentation and procedures for detecting geological anomalies in coal seams and more specifically to continuous-wave medium frequency radio imaging techniques combined with computer aided reconstruction to provide graphic radiogenic images of seam anomalies.
2. Description of the Prior Art
Coal seams or deposits occurring in layered formations have been distorted by many different types of geological mechanisms. Differential compaction occurring in surrounding layers causes faults, twists and rolls to occur in the seam. Ancient streams have washed coal from beds leaving sand and rock deposits. These deposits, known as fluvial channel sand scours, can cause washouts and weak roof. Such seam distortions and rock deposits are physical barriers to mining equipment. Two types of underground mining techniques are extensively used in the coal mining industry. One type, referred to as room and pillar, or continuous mining can mine around many of these barriers. The continuous mining technique is less expensive and requires less manpower. For example, set-up generally requires three shifts of eight people. Continuous mining, however, produces only approximately 300 tons per shift. Longwall mining, the other widely used technique, is much more efficient in uniform coal beds. This method yields production rates averaging 1500 ton per shift.
In the United States, the Mining Safety and Health Administration requires that retreating, rather than advancing longwalls be used. On the other hand, in Europe, advancing longwalls are extensively used. Retreating longwalls are set up to mine in the direction of the main entry, whereas advancing longwalls mine away from the main entry. Continuous mining techniques are employed to set up the retreating longwall. From the main entry, two entry ways are mined at right angles to the main entry and on either side of the longwall panel. These entry ways, the head-gate entry and tail-gate entry respectively, extend the length of the longwall panel. At the end of the panel, a crosscut is made between the head gate and tail gate entries. The wall of the crosscut facing the main entry is the longwall face. The longwall machine is set up along the face with a heading towards the main entry. As the longwall moves forward, the roof caves in over the mined out area. A barrier block of unmined coal is left at the end of the run to support the roof over the main entry.
The high yield of longwall mining makes it economically advantageous to use where a long panel can be mined. A typical longwall panel contains from 500,000 to one million tons of coal. The initial investment and set-up cost of longwall mining are high. Equipment cost averages many millions of dollars. Longwall set-up requires thirty days minimum, at three shifts per day with twelve to fourteen men per shift. Thus, set-up expenses are very large as a result and to achieve the low cost production advantage of the longwall method a uniform coal seam is necessary to ensure a long production run. Seam anomalies such as faults, washouts, interbeddings and dikes can cause premature termination of the longwall production run. In many instances, longwalls become "ironbound" after encountering an anomaly. Removal of such "ironbound" equipment requires blasting which can damage equipment and exposes miners to extreme danger. Accordingly, if seam anomalies could be detected and analyzed in advance of mining, the mining techniques could be planned for minimum production cost. Where the survey discloses a long continuous coal seam, the low cost longwall technique can be employed. If barriers to longwall mining are discovered the mine engineering department can use continuous mining to mine around the barriers, or the anomalies can be removed, for example by fracking or blasting.
Geological surveys for potentially productive coal formations use many well known procedures. These procedures employ a wide variety of technologies. Satellite imaging and photography provide global data for use by mine geologists. However, because of the broad overview of the data they are of no value in determining the mineability of a coal seam. Macrosurvey (foot prospecting) of surface strata and outcrop features enable geologists to forecast formation characteristics based upon prior knowledge. Surface based seismic and electromagnetic wave propagation procedures are extensively used in geophysical surveys for valuable deposits including oil and gas. These microsurvey techniques, however, are not reliable in examining the detailed structure of a coal seam.
Various microsurveying in-seam seismic techniques are currently employed to yield useful data concerning seam anomalies. A technique under development in Europe comprises firing shots from sixteen points into a block of 120 geophone groups, each consisting of thirty-six geophones. Computerized processing of the seismic data results in the detection of faults. To date, the procedure requires placing charges at five foot intervals and requires the installation of extensive cabling. Seismic techniques are primarily intended for advancing, rather than retreating longwalls. Further, this method has not proven to have the capability of resolving channel sand anomalies, especially for partial washouts and smaller, less significant anomalies, nor can they detect roof/floor rock conditions. The emerging of the surface based spectral magnetotelluric method with controlled sources may have the capability of seeing into the earth's crust. This method appears to be useful in detecting major faults in layered formations, but cannot resolve detailed seam structure.
Downhole drilling has been used to probe longwall blocks. A ten-twelve hole pattern drilled six-hundred feet into the panel provides samples of the coal in the seam. This method, however, has the disadvantage of covering only a small percentage of the block. Because of this limited coverage this technique is not useful to detect and resolve seam anomalies that may exist in the seam between the boreholes. Surface core drilling and logging remains the most reliable source of seam information. Core sampling provides useful data in mapping stratified mediums. Logging enables probing of the formation in the vicinity of the drill hole. None of the currently used logging methods can detect and resolve seam anomalies that may exist in the seam between the bore-holes over distances greater than about fifty feet. In-seam horizontal drilling can detect seam anomalies, but is subject to the same coverage limitations of vertical drilling. Horizontal drilling, additionally, is very expensive, averaging twenty cents per ton of coal produced.
Electromagnetic technologies have been investigated in an attempt to provide a geophysical method to see within the coal seams. Conventional and synthetic radar techniques have been reported in the literature. Because of the high frequency of the radar, it is exceedingly useful in investigating the geological structure in near proximity to the borehole. Deep seam penetration, however, requires very high transmit power in order to maintain any sort of useful resolution. This is because high frequency signals are attenuated very rapidly with distance in the seam. Accordingly, present radar methods cannot see deep into the seam.
Publications by R. J. Lytle, Cross Borehole Electromagnetic Probing to Locate High-Contrast Anomalies, Geophysics, Vol. 44, No. 10, Oct. 1979; and Computerized Geophysical Tomography, Proceedings of the IEEE, Vol. 67, No. 7, July 1979, have described a method of imaging coal seams using continuous wave (CW) signals. His method proposed only tomographic imaging between nearby boreholes. The method of Lytle had limited range and resolution, because of the limited spatial measurements that could be taken using downhole probes. To satisfy the requirements for tomography, Lytle used a higher frequency range, thus achieving less range. Further, the conductivity of rock was found to be much greater than the conductivity of coal. Where the difference conductivity (contrast) is large, the tomography algorithm will diverge rather than converge, resulting in no image.
A study conducted by Arthur D. Little, Inc. for the U.S. Bureau of Mines investigated continuous-wave medium-frequency signal propagation in coal. The results, published by Alfred G. Emslie and Robert L. Lagace, Radio Science, Vol. II, No. 4, April 1976, dealt with the use of electromagnetic waves for communication purposes only. Additionally, errors may be present in the wave propagation equations employed. United Kingdom Pat. No. 1,018,188, issued to Kaiser, discloses a method for testing various media utilizing high frequency radio waves. A well logging method and apparatus is disclosed in EPO Patent Application No. 0 105801, assigned to Schlumberger Limited. The method is not directed to deep seam penetration and imaging, but is used to obtain conductivity and dielectric measurements proximate to a borehole.
Other electromagnetic techniques suffer similar range and resolution problems. None of the prior art recognized the existance of a coal seam transmission window in the 300-800 kHz range. Accordingly, none of the prior art achieved a long range, high resolution imaging of geological anomalies.